Climate change has resulted in global increases in temperature and extreme heat events are projected to become more frequent and intense. Understanding health effects of extreme heat exposure is critical to developing mitigation strategies to protect those at greatest risk. Research has shown that extreme heat is linked to increases in mortality and has identified vulnerable, high-risk groups, including those with underlying cardiopulmonary disease, the poor, children, the elderly, and those living in urban areas and temperate climates. Far less is known about morbidity outcomes and very little is known about disease-specific outcomes, which are critical to estimate the healthcare burden of this exposure. To date, ambient temperatures have been used as the exposure metric, an approach subject to misclassification that may underestimate health effects in some groups. While there has been emphasis on individual level exposure assessment in environmental research, and this approach has been applied successfully to study air pollution, there has been little application to study heat-related health effects. We proposed to study the effect of individual exposure to extreme heat in vulnerable populations of inner-city minority children with asthma and older adults with COPD, using disease-specific outcome measures. We will leverage substantial resources from existing NIEHS-funded cohorts to address unique aims: 1. To determine a) the effect of individual exposure to extreme heat and b) the interactive effect of extreme heat and air pollution exposure on asthma morbidity among minority children with asthma living in inner-city Baltimore; 2. To determine a) the effect of individual exposure to extreme heat and b) the interactive effect of extreme heat and air pollution exposure heat on COPD morbidity among older adults with COPD living in Baltimore. Detailed participant characterization and daily activity diaries completed during heat events allow us to identify characteristics of those at highest risk within these vulnerable populations, information critical o development of mitigation strategies. In aim 3, we will model the relationship between ambient and individual heat exposure and identify factors that modify this relationship, which will inform exposure metrics in future population-level studies. We will, for the first time, use individual exposure assessment and disease- specific health outcomes to advance the understanding of health effects of extreme heat, projected to increase with climate change, on highly vulnerable populations with COPD and asthma.